This application relates generally to optical routing. More specifically, this application relates to passive optical networks.
Fiber-optic telecommunications systems are currently deploying wavelength division multiplexing (“WDM”) as a mechanism for expanding the capacity of new and existing optical fiber systems. In WDM, multiple wavelengths of radiation simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data.
A passive optical network (“PON”) is a communications architecture in which each single optical fiber transmits multiple channels of information in this way as distinct wavelengths of radiation. The network is termed “passive” because the wavelengths are combined (multiplexed) and separated (demultiplexed) using wavelength-selective splitters or couplers rather than active switching or variable routing components. One example of a PON is the distribution of video (television, cable TV, and/or video downloads), analog or digital telephone (local, long-distance, and/or VoIP), and digital computer data (Internet or network connectivity) from a service provider to a multiplicity of subscribers via optical fiber-to-the-premises (“FTTP” or “FTTx”) connections. At or near each subscriber/user location along such a PON, it is generally necessary to receive and demultiplex two or three incoming optical signals from a single optical fiber. This may be done at relatively wide wavelength spacing compared with the dense wavelength division multiplexing that is used for long-haul transmissions. At least on optical signal is multiplexed and transmitted back into the optical fiber at a distinct wavelength.
For example, in one implementation, downstream traffic is provided on a 1490-nm channel and upstream traffic is provided on a 1310-nm channel. In another implementation, an additional 1550-nm band is allocated for downstream rf video traffic.
There are a number of different PON standards in existence. The ITU-T G.983 standard defines standards for ATM PONs (“APON”) and broadband PONs (“BPON”). These represent the earliest PON standards, with BPON building on the APON standard to provide support for wavelength division multiplexing as well as better upstream bandwidth allocation. There has been more recent interest in the ITU-T G.984 standard, which supports higher rates and enhanced security as a further evolution of the BPON standard. In addition, the IEEE 802.3ah standard provides a standard for using Ethernet for packet data. Emerging approaches for providing customers with the “triple-play” of services described above over a common system or network require data-transfer rates in the gigabit/second (Gb/s) range or higher, which can be most easily achieved using optical rather than electronic (“copper”) transmission lines, leading to the nomenclature “GPON” for gigabit passive optical networks.
In implementing any of these various PONs, there is a general need for structures to manage the transmission of the different signals, both downstream and upstream. Current implementations suffer from a number of deficiencies. For example, persistent concerns with structures that are used exist in the isolation of the different wavelengths, the need to control the absorption of stray radiation, and the cost of fabrication.